1. Field of the Invention
The present invention relates to nuclear magnetic resonance for well logging. Specifically, the invention relates to a recent generation nuclear magnetic resonance well logging instruments using permanent magnets as a source of a static magnetic field.
2. Description of the Related Art
In exploration for hydrocarbons, it is important to make accurate measurements of geologic formations. The geologic formations below the surface of the earth may contain reservoirs of oil and gas. The geologic formations may include formation bedding planes and various structures. In a quest for oil and gas, it is important to know about the location and composition of the formation bedding planes and the various structures. In particular, it is important to know about the geologic formations with a high degree of accuracy so that drilling resources are not wasted. Measuring properties of the geologic formations provides information that can be useful for locating the reservoirs of oil and gas. Generally, the oil and gas are accessed by drilling boreholes into the subsurface of the earth. The boreholes also provide access for taking measurements of the geologic formations.
Well logging is a technique used to take measurements of the geologic formations from the boreholes. In one embodiment, a logging instrument is lowered on the end of a wireline into the borehole. The logging tool sends data via the wireline to the surface for recording. Output from the logging instrument comes in various forms and may be referred to as a “log.” One type of measurement involves using nuclear magnetic resonance (NMR) to measure properties of the geologic formations.
A recent generation of NMR well logging instruments use permanent magnets as a source of a static magnetic field (see U.S. Pat. Nos. 4,710,713; 4,717,877; 5,712,566; and 6,580,273). Prior art devices implementing NMR technology using the permanent magnets normally require high-energy magnetic material for the permanent magnet. One reason for using the high-energy magnetic material is to maximize the static magnetic field in a region of interest. In general, stronger static magnetic fields result in a higher signal to noise ratio (SNR) for an NMR signal or alternatively a deeper depth of investigation. Another reason for using the high-energy magnetic material is to provide stability of magnet magnetization in a high-temperature environment. For the high-energy magnetic material, a combination of a high coercivity and a small temperature coefficient of magnetization and coercivity is highly desirable to avoid irreversible loss of magnetization. The irreversible loss of magnetization will eventually cause demagnetization of the permanent magnet. Besides the fact that the permanent magnet is expensive, the permanent magnet also causes a problem resulting from a high attractive force when passing through a steel surface casing lining the borehole. Electromagnets that may solve the problem cannot operate as well as the permanent magnets in downhole applications. U.S. Pat. No. 4,717,877 discloses a magnet assembly with moveable parts that offers a solution for varying a magnetic field of a magnet assembly in order to perform radial imaging as well as for transporting the magnet.
Also known in the art are electrical and mechanical switchable magnets used for lifting magnetized objects. A system representing switchable magnets is disclosed, for example, in U.S. Pat. No. 6,229,422. A magnet assembly comprises two magnets with a coil around one of the magnets. The magnets are connected with a magnetically permeable frame. Energizing the coil in one direction reverses polarization of one magnet, thereby effectively “short circuiting” magnetic flux produced by the other magnet. The short circuiting of the magnetic flux terminates holding the magnetized object. Energizing the coil in the opposite direction causes parallel polarization of the magnets thereby switching the magnet assembly into a holding mode. The switchable magnet systems known in the art are not conducive for use in down-hole NMR measurements because of high energy storage requirements and an inability to provide complete zeroing of an external magnetic field.
What are needed are techniques for providing a switchable magnet having an energy storage requirement conducive for use in downhole NMR measurements with the switchable magnet providing zeroing of the external magnetic field.